Bright And Fast Neutron Scintillators

ABSTRACT

The invention pertains to a neutron detector comprising a scintillating material of the generic formula Cs(2−z)Rb z LiLn(1 −x )X 6 :xCe 3+ , where X is either Br or I, Ln is Y or Gd or Lu or Sc or La, where z is greater or equal to 0 and less or equal to 2, and x is above 0.0005, generally under the form of monocristal. The scintillating material has a remarkably low decay time.

The invention pertains to new scintillating materials, in particular in the form of single crystals, a process to manufacture them as single crystals, and their use for to detect and/or discriminate neutron and/or gamma ray radiations.

Scintillation materials are largely used for detection of gamma-rays (or electromagnetic waves of lower energies down to 1 keV or lower, below designated as “gammas”) as well as particles like neutrons, alpha particles etc.

The scintillation mechanisms rely on a number of physical principles which essentially convert the high energy of the incoming photons or particles into light which is within or reasonably close to the visible range. Of particular interest are single crystal forms of scintillators, ie pieces that are at the scale of use constituted of one (at most a few) crystals. A single crystal (monocristal) configuration allows easier extraction of the emitted light over thick dimensions thanks to lack of internal diffusion through grain boundaries, heterogeneities and other defects that tend to be present in polycrystalline materials. A crystalline structure (in the atomic sense) is required as it determines the scintillation mechanisms: a glassy, amorphous state of matter is likely to yield different scintillation properties. The extracted light is then collected with various devices well known to the man of the art, like photomultiplier tubes, photodiodes etc. Another configuration is to still retain the crystalline structure of the material, and use it in powder form, either packed or sintered or mixed with a binder in a way that still allows light extraction. Usually, those configurations are too opaque when more than a few millimeters thick, a thickness which may not be sufficient to stop enough incoming particles or photons. Overall, whenever possible and cost effective, single crystals are preferred.

Gammas, alpha or beta particles, charged particles, or neutrons (below designated generically as “radiation”) detection (is of major interest in a host of applications in nuclear medicine, fundamental physics, industrial gauging, baggage scanners, oil well logging etc. In those applications, it is often desirable to discriminate neutrons from gamma rays that may also reach the detector and a scintillation detector should be able to produce different luminescence signals depending on the type of radiation (see G. F. Knoll, Radiation Detection and Measurements (Wiley, New York, 2000)).

Several criteria are needed to constitute a good neutron or gamma detector.

In a way well know to the man in the field, an energy spectrum of the scintillator under incoming radiation is drawn, whereby events are represented on a histogram (with energies on the x-axis, the number of counts on the y-axis). In the acquisition protocol, “channels” are defined to collect the signal within a particular energy range.

Good (low) energy resolution is necessary for good energy peak identification of the incoming radiation. Energy resolution is usually determined for a given detector at a given energy as the full width at half maximum of the peak considered on an energy spectrum, divided by the energy at the centroid of the peak (see G. F Knoll, “Radiation Detection and Measurement”, John Wiley and Sons, Inc, 2^(nd) édition, p 114).

Another very important parameter is the decay time constant, which is described in particular by W. W Moses (Nucl. Instr and Meth. A336 (1993) 253). Fast decay times allow fast analyses. In general, the time spectrum of the collected signal from a scintillator under radiation (neutrons or gammas) can be fitted by a sum of exponentials characterized each by a decay time constant. The quality of a scintillator is determined essentially by the contribution of the fastest emission component. This is the number we report further in the text.

Neutron detection is often carried out with He3 tubes. This type of device has high detection efficiency, good neutron/gamma discrimination, can be made into a wide range of detector sizes, and has a long history in the field. Its weaknesses are a relatively long charge collection time and low signal levels. In a lot of cases, solid-state scintillators are preferred.

A number of scintillators used for neutron detection rely on the presence of ⁶Li in the molecule. ⁶Li is able to capture thermal neutrons and convert them into ionizing particles according to the reaction: ₃ ⁶Li+₀ ¹n→₁ ³H+α  (1) The α and triton particles share a kinetic energy of 4.78 MeV. The subsequent deposition of this energy in the material can lead to detectable luminescence. However the mere presence of ⁶Li does not guarantee good properties.

It is thus convenient to define a gamma/neutron discrimination indicator, the so-called γ-equivalent factor F_(γ) defined as: $\begin{matrix} {F_{\gamma} = {\frac{C_{n}}{C_{\gamma}} \cdot \frac{0.662}{4.78}}} & (2) \end{matrix}$ where C_(n) is the channel of the neutron peak (in the acquisition protocol) and C_(γ) the channel of the γ peak while 0.662 MeV is the energy of the γ-rays and 4.78 MeV the kinetic energy of the α and triton particles together (resulting from neutron capture).

For radiography, LiF+ZnS(Ag) screens are quite standard. The high light output (160 000 photons/neutron) is very attractive and allows good sensitivity. Its decay time is in the microsecond range.

Li glass, especially the kind enriched in ⁶Li, is another prominent neutron detector. Its light output is unfortunately very low at 6000 photons/neutron and its energy resolution high. It has the advantage of fast decay, with a decay constant at about 75 ns.

⁶Lil (lithium iodide) doped with Europium (Eu) is also a known neutron detector. The light output at about 51 000 photons/neutron is very good. The energy resolution is nonetheless extremely high, the decay constant as high as that of LiF+ZnS(Ag).

Of late, new compounds have been discovered by Bessiere et al (Scint 2003, to be published in proceedings), namely Cs₂LiYCl₆ and Cs₂LiYCl₆:Ce³⁺. Both exhibit high light output (34 000 and 70 000 respectively). Their γ-equivalent factor is also very good at 0.61 and 0.66 respectively. All results on the Ce-doped version are given for a concentration of 0.1 mol %. Nonetheless, those two compounds have slow decay times (about 6-7 microseconds).

In addition, Cs₂LiYBr₆ and Cs₂LiYBr₆:Ce are mentioned in “Scintillation and Spectroscopy of the Pure and Ce³⁺-Doped Elpasolithes”, Van Loef et al., Journal of Physics Condensed Matter, 14, 2002, 8481-8496. The optical and scintillation (under gamma-rays) properties of these compounds are investigated. There is however no reference to their scintillation characteristics under neutrons. Only the fact that Li presence may make such bromine-based compounds attractive is mentioned. This does not guarantee good performance under neutrons.

Present invention pertains to a new material showing a remarkably high light yield and a fast (i.e. low) decay time to neutrons (which can be designated by τ). The material also preserves good detection properties for gamma-rays and allows good discrimination between gamma-rays and neutrons.

The material of the invention has as generic formula Cs_((2−z))Rb_(z)LiLn_((1−x))X₆:xCe³⁺, where X is either Br or I, Ln is Y or Gd or Lu or Sc or La, where z is greater or equal to 0 and less or equal to 2, and x is above 0.0005. The value x is the molar fraction of the sum of Ln and Ce. The value x is above 0.0005, but it can also be said that x is above 0.05 mol %. Both wordings are equivalent. Generally, x is less than 0.3 (=less than 30 mol %) and more generally less than 0.15.

Particularly useful compositions are:

-   -   Cs₂LiY_((1−x))X₆:xCe³⁺,     -   CsRbLiY_((1−x))X₆:xCe³⁺.

The scintillating material of the invention can be manufactured under the form of a monocristal. In this case, its volume is generally at least 10 mm³. The material can also be used in the form of a powder (polycristalline), possibly either packed or sintered or mixed with a binder.

The compounds were grown as single crystals by the vertical Bridgman technique. Because the compounds are hygroscopic the experiments were performed on samples sealed in quartz ampoules. Other known techniques to the man in the field of crystal growths could be used such as Czochralski or Kyropoulos techniques, provided the material is reasonably protected from reaction with water and oxygen. The single crystal samples in the examples were about 10 mm³ in volume. They contain a natural abundance of ⁶Li, but they could be advantageously enriched in it (to the detriment of cost).

The properties of Cs₂LiYBr₆:Ce³⁺ are given in Table 1 below, along with those of the scintillators already known in the field of neutron detection. TABLE 1 Comparative properties of Cs₂LiYBr₆:Ce³⁺ and other neutron scintillators. Light Yield Energy Pulse Height Decay Time (μs) (photons/ resolution to Discrimination Under gammas/ Compound neutron) neutrons (%) F_(γ) neutrons 1 Cs₂LiYCl₆ 34000 14.5 0.61 7.0 2 Cs₂LiYCl₆:0.1% Ce 70000 5.5 0.66 6.5 3 Cs₂LiYBr₆:1% Ce 85800/88200 4.5/9.0* 0.76/0.75 0.089 4 Cs₂LiYBr₆:0.3% Ce 73 000 — — 0.070/0.0083 5 Rb₂LiYBr₆:0.5% Ce 65 000 — — — 6 LiF—ZnS:Ag 160 000  — 0.45 1 7 Lil:Eu 51 000 40 0.86 1.2 8 Li glass  6 000 13-22 0.31 0.075 (*Double-peak structure in spectrum for sample) (0.1% Ce significates x = 0.001 in the generic formula which is in fact Cs₂LiY_(0.999)Ce_(0.001)X₆.)

It appears from Table 1 that the samples according to the invention (samples 3, 4, 5) have excellent light yields to neutrons, fast decay times and good gamma-neutron discrimination as seen from the F_(γ) factor. The emission wavelength has a double peak structure at 390 and 420 nm approximately, which conveniently matches the maximum sensitivity of bi-alkali photomultiplier tubes. The products according to the invention have the advantage over Cs₂LiYCl₆:0.1% Ce (as in sample 2) to be extremely fast scintillators, and to have even higher light outputs and F_(γ) factors. These features are not described in the Van Loef et al., reference mentioned above. Table 1 also shows that Rb₂LiYBr₆:Ce according to the invention (with a complete substitution of Cs by Rb) is equally interesting. Finally, although not described here, substitution of Y by other rare earths or assimilated rare earths in the same column in the periodic table like Sc, La, Lu is also part of the invention.

The following Table 2 summarizes in more detail the properties of the compounds of the invention in the cases of gammas (as emitted by a ¹³⁷Cs source), neutrons alone or combined gammas+neutrons detection. The first two decay constants (the fastest, then second fastest) in the exponential series decomposition are reported. TABLE 2 Main characteristics of Cs₂LiYBr₆:Ce³⁺ under gamma, neutron and gamma + neutrons radiations (R_(n) = energy resolution for neutron) ¹³⁷Cs γ + ¹³⁷Cs γ-rays Thermal neutrons neutrons LY R LY R_(n) τ Compound (ph/MeV) (%) (ph/n) F_(γ) (%) (μs) Cs₂LiYCl₆ 11 700 12.0  34 000 0.61 14.5  0.004/7.0 Cs₂LiYCl₆:0.1% 21 600 8.0 70 000 0.66 5.5 0.004/non Ce exp. Cs₂LiYBr₆:1% 23 600 7.0 85 800 0.76 4.5 0.089/2.5 Ce 24 700 8.5 88 200 0.75 9.0 id. It is shown in particular that materials of the invention constitute an interesting gamma detectors, less bright than the NaI:TI known in the field of gamma detection, equivalent Energy resolution but much faster decay. 

1: A neutron detector comprising scintillating material Cs_((2−z))Rb_(z)LiLn_((1−x))X₆:xCe³⁺, where X is either Br or I, Ln is Y or Gd or Lu or Sc or La, where z is greater or equal to 0 and less or equal to 2, and x is above 0.0005. 2: The neutron detector according to claim 1, wherein x is above 0.005. 3: The neutron detector according to claim 1, wherein x is less than 0.3. 4: The neutron detector according to claim 1, wherein x is less than 0.15. 5: The neutron detector according to claim 1, wherein it is in the form of a monocrystal. 6: The neutron detector according to claim 5, wherein the volume of the monocrystal is at least 10 mm³. 7: The neutron detector according to claim 1, wherein it is in the form of a powder. 8: The neutron detector according to claim 1, wherein it is packed, sintered, or mixed with a binder. 9: The neutron detector according to claim 1, wherein its formula is Cs₂LiYX₆:xCe³⁺. 10: The neutron detector according to claim 1, wherein its formula is Rb₂LiYX₆:xCe³⁺. 11: A method of neutron detection comprising detecting neutrons using a material of formula Cs_((2−z))Rb_(z)LiLn_((1−x))X₆:xCe³⁺, where X is either Br or I, L is Y or Gd or Lu or Sc or La, where z is greater or equal to 0 and less or equal to 2, and x is above 0.0005. 12: The method according to claim 11, wherein x is above 0.005. 13: The method according to claim 11, wherein x is less than 0.3. 14: The method according to claim 13, wherein x is less than 0.15. 15: The method according to claim 11, wherein the material is in the form of a monocrystal. 16: The method according to claim 15, wherein the volume of the monocrystal is at least 10 mm³. 17: The method according to claim 11, wherein the material is in the form of a powder. 18: The method according to claim 17, wherein the material is packed, sintered, or mixed with a binder. 19: The method according to claim 11, wherein the material formula is Cs₂LiYX₆:xCe³⁺. 20: The method according to claim 11, wherein the material formula is Rb₂LiYX₆:xCe³⁺. 21: A material of the formula Rb₂LiLnX₆:xCe³⁺ where X is either Br or I, Ln is Y or Gd or Lu or Sc or La, and x is above 0.0005. 22: A material of the formula Cs_((2−z))Rb_(z)LiLn_((1−x))X₆:xCe³⁺, where Ln is Y or Gd or Lu or Sc or La, where z is greater or equal to 0 and less or equal to 2, and x is above 0.0005. 23: The material according to claim 21, where x is above 0.005. 24: The material according to claim 21, wherein x is less than 0.3. 25: The material according to claim 24, wherein x is less than 0.15. 26: The material according to claim 21, wherein it is in the form of a monocrystal. 27: The material according to claim 26, wherein the volume of the monocrystal is at least 10 mm³. 28: The material according to claim 21, wherein it is in the form of a powder. 29: The material according to claim 28, wherein it is packed, sintered, or mixed with a binder. 30: The material according to claim 21, where Ln is Y. 